1. Field of Disclosure
This application relates generally to current sense transistors and, more specifically, to techniques to compensate for variations in the current sense ratio between a current sensing transistor and a main transistor.
2. Description of the Related Art
Current sense transistors have been used for many years in integrated circuit applications where accurate current sensing can provide information for both control and over-current protection. Sense transistors are typically constructed from a small part or section of a larger transistor that carries the main current of the device. For example, in a conventional metal oxide semiconductor field effect transistor (MOSFET) device, the sense transistor may comprise a small section of the channel region of the main power transistor. In operation, the sense transistor may sample a small fraction of the channel current of the power transistor, thereby providing an indication of the current in the main transistor. The sense transistor and main transistor device typically share a common drain and gate, but each has a separate source electrode.
Sense transistors are useful in many power delivery applications to provide current limit protection and accurate power delivery. In providing these functions, the sense transistor generally maintains a constant current sensing ratio (CSR) with respect to a main power transistor over a wide range of drain currents (100 mA to 10 amperes), temperatures (−25° C. to 125° C.), as well as fabrication process variations and mechanical stress/packaging variations. The ratio of drain current of the main power transistor to that of the sense transistor typically ranges between 20:1 to 800:1, or greater.
High electron mobility transistors (HEMTs) are attractive devices for achieving high performance in high power applications as they have high electron mobility and a wide band gap, and are capable of being processed with conventional equipment and methods not substantially different from those already developed for silicon and present generations of compound semiconductors. A particularly desirable material for building a HEMT is the wide-bandgap compound semiconductor known as gallium nitride (GaN). The GaN-based transistor is capable of maximizing electron mobility by forming a quantum well at the heterojunction interface between e.g., an aluminum gallium nitride (AlGaN) barrier layer and a GaN layer. GaN-based transistors have received much attention for high power applications since they have on-resistances that are typically one or more orders of magnitude less than those of silicon (Si)-based or gallium arsenide (GaAs)-based transistors and hence, are operable at higher temperatures with higher currents and can withstand high voltage applications.